Magnetic dipole decays of heavy and heavy-light mesons

The widths of M1 transitions in heavy quarkonia are shown to be sensitive to two-quark exchange current operators that depend explicitly on the Lorentz structure of the quark-antiquark interaction. A possible solution to the long-standing overprediction by a factor similar to 3 of the width for J/psi --> eta(c)gamma obtains, if the effective confining interaction has scalar Lorentz structure. The nonrelativistic predictions for the M1 widths of the Q (q) over bar mesons are shown to be unrealistic, as the confined light constituent quark requires a relativistic treatment. However, as the form of the Q (q) over bar interaction is uncertain, the results should be viewed as suggestive.